The merozoite asexual stage of Cryptosporidium multiplies in logarithmic fashion during infection and is largely responsible for the overwhelming parasite burden encountered in many AIDS patients with persistent cryptosporidiosis. Unfortunately, there are no current effective chemotherapeutic treatment regimens to control this infection. The main objective of this project, therefore, will be to design an immunoprophylactic and immunotherapeutic treatment strategy for controlling cryptosporidiosis based on the development of infectivity neutralizing monoclonal antibodies (MAbs)to the merozoite stage. We plan to test the hypothesis that this approach, which may be supplemented with monoclonal antibodies developed to other life cycle stages of this parasite, may ultimately provide a realistic and reproducible means of treating persistent cryptosporidiosis in AIDS and other immunocompromised patients. Specific approaches to be taken to accomplish this objective will include: 1) producing and testing the antiparasitic neutralizing activity of murine monoclonal antibodies reactive with stage-specific merozoite antigens, 2) identifying and purifying relevant merozoite antigens using neutralizing MAbs, 3) defining optimal cell culture condition for the in vitro cultivation of Cryptosporidium so this system can be used to identify additional neutralizing MAbs or potentially effective chemotherapeutic agents, 4) construction of Cryptosporidium cDNA and genomic libraries to assist in identifying or producing antigens recognized by neutralizing MAbs, and 5) developing bovine-murine heterohybridomas to relevant parasite antigens as an alternative immunoprophylactic or immunotherapeutic treatment strategy.